The present invention relates to an image forming apparatus for forming a color image by electrophotography.
Some image forming apparatuses include a photosensitive belt for carrying a latent image and a plurality of developing units that are arranged opposite to the photosensitive belt to selectively supply toners of different colors (Y, M, C, K) to the belt.
In the image forming apparatuses, toner images of different colors formed on the photosensitive belt are superimposed and transferred to an intermediate transfer body to form color images. After that, the color images are transferred to recording paper at once.
In the image forming apparatuses, the developing unit for each color moves between a developing position in which its developing roller contacts the photosensitive belt and a retreating position to which the developing roller retreats from the developing position. In contact monocomponent development wherein development is performed by a developing unit filled with toner only, a toner thin film is formed on the developing roller and brought into contact with a latent image on the photosensitive belt in the developing position. After the development, the developing roller separates from the photosensitive belt and moves to the retreating position.
In the monocomponent development, a toner layer formed on the developing roller is moved to a latent image on the photosensitive belt by electrostatic force, and the latent image is developed. In this case, almost all the toner moves to the photosensitive belt from a pixel portion of the developing roller and thus the toner hardly remains on the developing roller after the development.
Toner should therefore be supplied to the developing roller to form a uniform toner layer until the developing roller makes one rotation in the developing unit and reaches a developing area again.
However, the above image forming apparatuses have the problem that an adequately thick toner layer cannot be formed continuously for an image having a high printing rate, especially an image like a belt lengthened in the sub-scanning direction and its density decreases gradually.
The present invention has been made in consideration of the above situation and its object is to provide an image forming apparatus that is capable of continuously forming a sufficiently thick developer layer even though an image has a high printing rate or an image is shaped like a belt lengthened in the sub-scanning direction.
An image forming apparatus according to a first aspect of the present invention, comprises a rotatable image carrying body which carries an electrostatic latent image, an image forming device which forms the electrostatic latent image on the image carrying body, a plurality of developing devices which selectively supply developers of different colors to the electrostatic latent image formed by the image forming device by rotation of developing rollers and develop developer images of different colors, a driving device which rotates the developing rollers, a moving device which selectively moves the developing devices between a first position in which the developing rollers contact the image carrying body and a second position in which the developing rollers separate from the image carrying body, a transfer device which superimposes and transfers the developer images of different colors developed by the plurality of developing devices onto a medium, and a control device which rotates the developing rollers at least one revolution in the second position before one of the plurality of developing devices moves to the first position and develops a latent image.
An image forming apparatus according to a first aspect of the present invention, comprises a rotatable image carrying body which carries an electrostatic latent image, an image forming device which forms first and second electrostatic latent images on the image carrying body at regular intervals, a plurality of developing devices which selectively supply developers of different colors to the electrostatic latent images formed by the image forming device by rotation of developing rollers and develop developer images of different colors, a driving device which rotates the developing rollers, a moving device which selectively moves the developing devices between a first position in which the developing rollers contact the image carrying body and a second position in which the developing rollers separate from the image carrying body, a transfer device which superimposes and transfers the developer images of different colors developed by the plurality of developing devices onto a medium, and a control device which rotates the developing rollers when an area having no latent images between the first and second electrostatic latent images passes the developing rollers in the second position before one of the plurality of developing devices moves to the first position and develops a latent image.
An image forming apparatus according to a third aspect of the present invention, comprises a rotatable image carrying body which carries an electrostatic latent image, an image forming device which forms the electrostatic latent image on the image carrying body, a plurality of developing devices which selectively supply developers of different colors to the electrostatic latent image formed by the image forming device by rotation of developing rollers and develop developer images of different colors, a driving device which rotates the developing rollers, a moving device which selectively moves the developing devices between a first position in which the developing rollers contact the image carrying body and a second position in which the developing rollers separate from the image carrying body, a transfer device which superimposes and transfers the developer images of different colors developed by the plurality of developing devices onto a medium, and a control device which rotates the developing roller of at least one of the developing devices, which is separated from the image carrying body, simultaneously with rotation of the developing rollers of the developing devices that develop a latent image on the image carrying body.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.